Aperture-controlled electrostatic printing system and method employing ion projection

ABSTRACT

Methods and apparatus for electrostatic printing including a multi-layer screen having a conductive layer and an insulative layer and having apertures therein, means for deploying opposite electrostatic charges across the insulative layer, an image projecting system for modulating the charge on the insulative layer in accordance with light received thereby, a Corotron or other like means for projecting ions through unblocked apertures in the screen and through partially blocked apertures in the screen in fewer numbers to a substrate for subsequent developing and fixing to provide both positive and negative reproductions.

United States Patent 1191 McFarlane, Jr. et a]. 1 May 6, 1975APERTURE-CONTROLLED [56] References Cited ELECTROSTATIC PRINTING SYSTEMAND UNITED STATES PATENTS METHOD EMPLOYING ION PROJECTION 3,339,4699/1967 McFarlane.............................. 355/3 [76] Inventors:Samuel B. McFarlane, Jr., 11

Twombley Dr., Summit, NJ. 07901; Joseph Burdige, 17 Nansen Ct., SpringValley, N.Y. 10977; Norman E. Alexander, 24 Morris Ln., Scarsdale, N.Y.10583 [22] Filed: Dec. 12, 1973 [21] Appl. No.: 423,984

Related U.S. Application Data [60] Continuation of Ser. No. 167,011,July 28, 1971, which is a division of Ser. No. 709,578, March 1, 1968,Pat. No. 3,645,614.

[52] U.S. Cl 355/17; 355/3 [51] Int. Cl G03g 17/00 [58] Field of Search355/3, 17; 96/1 SINGLE SIGN ll ACTUAL BLOCKED AREA III PrimaryExaminer-John M. Horan Attorney, Agent, or FirmWilfred G. Caldwell, Esq.

[5 7] ABSTRACT 19 Claims, 10 Drawing Figures /SJBSTRATE TO BE PRINTEDFARE/k roeemrmeo a o. J

ill/I PATENIEDmw 61975 3.881.818

sum 10$ 6 L '/5UBSTRATE To 55 PRINTED kmTOE 6i ACTUAL BLm ED AREA 17 4DESIRED BLOCKED 2 I it INK PARTICLES [TI TOR 2| FIG 2 PHOTOSENSITIVEMATERIAL HIGH STRENGTH INSU LATm CONDUCTOR FIG .40

HIGH RESISTIVITY h 2 MATERIAL PI-IOTOSENSITIVE MATERIAL CON DUC TOR FIG.4b

THIN LAYER PHOTOSENS T E MATERIAL INSULATOR CONDUCTOR FIG .4c

k E Ill) PATENIEDHM 61975 SHEET 3 0F 6 mUmDOm mmZOk OP PATENIEDHAY 819.5

SHEET [If 6 m ul mmmiomluzkw PATENTEUIIAY M975 SHEET 5 BF 6 mwN j (EbmZmmE mmEZOm PATENTEDHAY s 1975 SHEET 8 BF 6 APERTURE-CONTROLLEDELECTROSTATIC PRINTING SYSTEM AND METHOD EMPLOYING ION PROJECTION Thisis a continuation of application Ser. No. 167,01 1 filed July 28, 1971which is a Divisional of U.S. Pat. No. 709,578 filed March 1, 1968 nowUS. Pat. No. 3,645,614 issued Feb. 29, 1972.

This invention relates to an aperture-controlled electrostatic printingprocess and method which employs a multi-layer screen comprising atleast a conductive layer and a superimposed layer to enable thedeployment of opposite electrostatic charges on the screen across theinsulative layer. The double layer charges are modified in accordancewith an image to produce blocking and non-blocking fields controllingthe apertures in accordance with the image to be reproduced. Theconductive screen layer is maintained at a potential (sometimes ground),and a propulsion field is provided for directing ions toward and throughthe screen. The ions pass through the screen where the apertures are notblocked by the fringing fields; they also pass through apertures whichare partially blocked, but in fewer numbers. This process uses thefringing field pattern of the apertures which modulates the flow of ionsthrough the screen to an insulative receiving medium, via preferably anair gap, for subsequent developing and fixing thereon, if necessary, bya conventional technique, such as for example, a liquid or a solidtoner, an aerosol cloud, a magnetic brush, or the like.

The insulative layer of the screen may comprise a photoconductor whichis merely charged or discharge in accordance with a light pattern, or itmay comprise an insulator other than of the photoconductive type whichmay be electrically charged. Alternatively, if the selected insulatorscreen has a low dielectric strength, a thin undercoating of a highdielectric strength material, not necessarily photoconductive, isemployed between the photoconductive layer and the conductive layer.Similarly, a thin overcoating of high resistivity material may beemployed to provide a charged carrier for photoconductors with poorsurface resistivity. When employing photoelectric materials that cannotbe deposited in heavy layers, the insulating layer may be comprised ofany good insulating material which will accept the sensitive material asa thin deposit. Thus, a thin layer photosensitive material may be coatedover the screen comprised of an insulative layer and a conductive layer.

Other materials which may be used as the insulator layers arephotoemissive material, polyester films, epoxy, photoresists, fusedquartz, or combinations thereof. In addition, the conductor backingitself may be deposited on the insulator, or a separate insulator layernot taking part directly in the electrostatic process may be used tosupport both the conductor and insulator layers.

The receiving medium may comprise paper or other materials, preferablycoated with a very thin layer of plastic or other flexible insulativematerial, such as polystyrene, polyvinyl chloride, cellulose acetate,such thin layer coated paper being commerically available at the presenttime.

The present invention improves over the known stencil type inventions,such as disclosed in US. Pat. No. 3,061,068 to C. O. Childress et a1.issued Mar. 16, 1963, and entitled Electrostatic Printing System for thereason that the screen employed in this patent must be in the form of apermanent stencil having openings where printing is desired and throughwhich charged particles pass to the print receiving material. Thesestencils, however, are not useful for producing more than one shape ofimage without resorting to stencilforming processes to change the image.Such stencilforming process may be similar to the production of asilkscreen image.

In the present invention, the screen is instantly reuseable; there is nophysical stencil required; and no problems of any buildup of tonerparticles on the screen are encountered.

The present invention differs from the inventions disclosed in US. Pat.3,220,831 to Sameul McFarlane issued Nov. 30, 1965, and entitledElectrostatic Printing Method and Apparatus Using Developer PowderProjection Means, and also US. Pat. 3,220,833 to Samuel McFarlane issuedNov. 30, 1965, entitled Electrostatic Printing Method in that theMcFarlane inventions employ electrostatic latent images which arepowdered and the power image is projected across an air gap from aphotoconductive needle tip carrier in the former patent or from aphotoconductive coated screen carrier in the latter patent.

The present invention actually electrostatically modulates the aperturesof the screen through the provision of the double layer charge, which ismodified in accordance with the image, to control the flow of ionsthrough the screen to the print-receiving material where a chargepattern is built up in accordance with the image to be reproduced, suchthat development of the charge pattern results in production of avisible image.

In the composite screen structure of the present invention, theconductive layer at fixed potential or ground performs two functions. Inthe first place, it enables the double layer charge to be establishedacross the insulative layer, thereby developing the fringing or blockingfields within the apertures of the screen, which fields are subsequentlymodulated in accordance with the image pattern. In the preferredembodiment, it enables the maintenance of the blocking fields duringprojection of the ions; the charges of the ions which do not passthrough the grid are simply dissipated due to the electrical potentialmaintained at the conductive layer.

The conductive layer may also be used to establish an electrical fieldbetween the screen and receiving material, if this is desired. Themagnitude of this field can be used to control the fraction of emittedcharge (number of ions) which reach the receiving medium.

Thus, the invention may comprise a composite screen mounted for endlessmovement and having at least an insulative layer and a conductive layerwith coinciding mesh. The composite screen is uniformly charged. Animaging station is provided for exposing the charged screen. When aphotoconductor is employed as the insulative layer of the screen, such amaterial is an insulator in the dark and becomes conductive upon lightimpingement. It can be charged by ions sprayed from an electrode, and alight image is then used to discharge those areas to be printed. Thelight image is reproduced in negative form because printing occurs wherethe image light impinges on the screen and the discharge has beendiminished or reduced to zero. For positive printing, the screen may becharged by an applied (internal) field during exposure to the lightimages. Illuminated areas of the screen photoconductive layer becomeconductive and under the influence of the applied field cause a chargeseparation similar to the double layer charge previously mentioned.After the charge separation is formed, the illumination is removed,causing all parts of the screen photosensitive layer to becomeinsulative. Then the charging field is removed; the portions of thefield which were illuminated remain charged and thus block the passageof the ions during the pattern buildup on the viewing material.

The ions from the latter source neutralize the oppositely charged ionswhere they exist on the screen, but pass through where they have beenneutralized or diminished by the light exposure. Timing is critical inthis approach, because the exposure should be limited in time so that itonly takes off the charges where the light impinges.

Of the photoconductor materials, it is known that selenium willpreferentially receive a positive charge and zinc oxide willpreferentially receive a negative charge. Some materials, however, suchas a mixture of cadmium sulfide and zinc sulfide and others, willreceive either a positive or a negative charge; it is from this group ofmaterials that the insulative layer of the screen is preferablyselected.

In the ease of negative printing as in microfilm exposures, themodulated apertures of the screen, depicting the image area, move intoan ion propulsion field where ions are projected toward the screen andpass through the screen in accordance with the modulation to continueacross an air gap, due to the propulsion field, to flexible plasticcoated print receiving paper. The charge pattern created by the ions onthe nonconductive paper is then developed with, for example, charged inkparticles. A heat fixing station fixes the ink, where necessary, becausethis process may employ powdered inks, as well as aerosol sprays orliquid droplets.

With the foregoing in mind, it is among the objects of the invention toprovide an aperture-controlled electrostatic printing process and methodwhich enable printing through a modulated screen onto nonconductivepaper or other substrate, across an air gap.

It is a further object of the invention to provide such reproductionsimulating half-tone printing with varying degrees of gray-to-blackprinting or sequential color reproduction.

A further object of the invention is the provision of the coordinationincluding the novel arrangement of a multi-layer screen susceptible toimage modulation for controlling the passage of ions therethrough.

Another important object of the invention is the provision ofreproduction methods and apparatus wherein no charged marking materialis propelled against or through any of the components, thereby avoidingsuch overall problems as clogging of apertures, transport to and throughthe screen, and removal of excess or unused marking material.

It is a further object of the invention to provide a method wherein adouble layer charging of a screen may be employed for subsequentmodulation or to provide blocking fields to ions in the apertures ofblank areas of the image being reproduced.

Yet another object is the provision of positive or negative printingfree of holidays and with good edge effects.

The invention will be better understood from a reading of the followingdetailed description thereof, when taken in conjunction with the drawingwherein:

FIG. I is a prior art arrangement to depict single charge stencil typeblocking of charged toner particles with fringe effects;

FIG. 2 is a view in section of a preferred embodiment of the screen ofthe present invention;

FIG. 3 is an enlarged view of a portion of FIG. 2;

FIG. 4a is a view, in cross section, of a portion of a screen showingthe use of low dielectric strength photosensitive material inconjunction with high dielectric strength insulative material placedbetween the photo sensitive layer and the conductive layer;

FIG. 4b is a similar view showing the use of high resistivity materialas the charge carrier overlying photosensitive material with poorsurface resistivity;

FIG. 4c is another view employing a conductive layer, a good insulativelayer, and a thin layer of photosensitive material deposited over theinsulator and within the apertures;

FIG. 5 is a schematic arrangement of an apparatus which uniformlycharges the screen, exposes, projects ions through the modulatedapertures, and develops and fixes the charge pattern on the printreceiving material;

FIG. 6 shows a similar apparatus but wherein scanning or modulation maybe effected almost simultaneously with, but just prior to, projectionsof the ions for positive printing;

FIG. 6a illustrates apparatus for another method of positive printingwherein the applied field of the screen is modified in accordance withthe double charge principle; and

FIG. 7 shows a further apparatus suitable for sequential charging,exposing, projecting of ions, developing and fusing similar to theapparatus of FIG. 5 but using differing polarities therefrom.

In FIG. 1 there is shown a prior art arrangement for stencil blockingutilizing a single sign charge layer only to show the limitations ofthis approach. The substrate 15 to be printed is positioned behind thestencil 17 which is positively charged, and the charged ink particles ortoner material 19 are similarly charged and projected toward thesubstrate.

Electrostatic printing is normally achieved by the propulsion of thecharged ink particles 19 through the fixed stencil 17 by means of anelectric field. The blocked portions of the stencil l7 prevent passageof certain of the ink particles 19, thus forming the image that isprinted. This used of mechanical blocking requires that the stencils beprepared by mechanical or photochemical means; these are slow processes,requiring several hours for the completion of a screen stencil.

Greater usefulness of the electrostatic printing process is achieved ifthe stencils could be substituted for and the substitute prepared withinseconds and if the image could be erased and the screen reused.

As is well known, the presence of a concentration of charges will createsurrounding fields such that the charges of like sign are repelled fromthe charged area. It is clear that if an image is formed of coplanarunifomily charged layers and the sign of the charges used to form theimage is the same as the charge on the toner particles, the toner willbe repelled from the charged areas, thus producing the blocking requiredto use the image as a stencil. Since this blocking of the passage of thecharged toner or equivalent is accomplished by the field surrounding thecharge layer, these fields are called blocking fields."

A one sign charge layer, however, will not satisfy the requirements of ablocking field, since the fields of such a system extend in alldirections from the charges. Thus, toner particles will be repelled notonly from the surface of the charge layer (the desired blocking effeet), but also from the edges of the charge layer which exist at theimage boundaries (FIG. 1). For printing to occur, particles must passthrough the uncharged areas (indicated in FIG. 1 as AREA TO BE PRINTED).The lateral repulsion field existing at the edge of the layer increasesthe blocking area, diffuses the edges of the printed image, and preventspassage of ink through small gaps in the charge layer.

The present invention overcomes the in the described above whilepermitting the desired charge layer blocking inthe nonprinting areas ofthe image.

The screen used to carry the charges and the disposition of charges onthe screen so as to perform the blocking action relative to ions 20,thus forming an electric charge pattern, are illustrated in FIG. 2. Thescreen is constructed of conventional insulator material 21 layered witha conductor 23; the holes 25 through which the ions pass extend incoincidence through both layers of the screen.

Electrical connection is made to the conductor layer 23 of the screen bytab 31 and lead 33 so that the potential of the backing members can bemaintained at, for example, ground or other desired potential duringcharging, exposure, and ion projection.

The insulator portion is charged so as to acquire a double layer" ofcharge (as indicated in FIG. 3) in which one face of the insulator 21contains Charges of one polarity, while the other surface contains anequal amount of charge of opposite polarity. (The charge layer which isformed on the insulator surface in contact with the conductor appears onthe surface of the conductor 23 as shown in FIG. 3.) Thus, the nextcharge on the screen is zero; therefore, no field exists from thesecharges at a distance of more than a few screen thicknesses away fromthe charged double layer. The motion of ion particles which have passedthrough the screen at uncharged areas is therefore not affected by thecharged areas of the screen.

Charging of the form indicated in FIG. 3 is made possible by thepresence of the conductor layer. A charge source, such as a corona wand,Corotrons, or radioactive strip, is used to spray ions on the surface ofthe insulator; the conductor portion of the screen is maintained at afixed potential (or ground) during this process so that any charge whichdeposits on the insulator surface will attract an equal and oppositecharge to the junction between the insulator and the conductor, thuscreating the required double layer.

Blocking of ions in the charged areas is performed by the fringingfields which exist within the holes of the screen. The basic fringingfield is oriented so as to prevent ions from passing through a hole. Theelectrical field lines within the apertures are such that the positivelycharged ions will be deflected to one or the other sides of an apertureand are collected and the charge disseminated by the conductor 23.

If the ions 20 are positive, then the double layer charges are arrangedso that the particles approach the screens negatively charged side;conversely, negative ions must be directed toward the positively chargedsurface.

The weakest fringing field exists along the center of the hole, and themagnitude of this field depends on both the charge magnitude (strengthof the field inside the insulator) and the thickness-to-diameter ratio(I /D) for the screen to aperture. Since the fringing field increases instrength as the insulator thickness increases, it is clear that foreffective blocking a large ratio of T,/D as well as high charge level isdesirable. The amount of fringing field required to block the ionsdepends on the strength of the field used to propel the ions from thesource to the printing substrate, i.e., the relative Corotron potentiallevel, spacing and any additional propelling fields. If the ions had noinertia. blocking would occur if the combination of fringing field andthe propulsion field, which act in opposition, produce a net zero fieldor repulsive field at any point along the centerline of the hole. Ioninertia effects, however, will carry the ion through the hole unless thecombined fields within the hole exert a net repelling force.

Since no charged marking materials are projected toward or through thescreen, it is unnecessary to be concerned over the collection of suchmarking material on the screen. In addition, the need for providingmetering and distributing devices is obviated.

To obtain printing, the charge image on the screen in at least oneembodiment must be the negative of the desired print, i.e., printingwill occur where no charge or reduced charge exists. A number oftechniques may be used to create the charge image.

The preferred technique is the utilization of a photoconductive materialas the insulator layer of the screen for either type printing. Such amaterial, which is an insulator in the dark and becomes conductive inthe light, can be charged as described above, e.g., with a corona wand,and a light image used to discharge those areas to be printed. Thus, thelight image would be reproduced in negative form.

The preferred embodiment of the invention relates to positive printing,and particularly to FIGS. 6 and 6a. In FIG. 6 the screen is charged byan ion source and discharged by the image prior to or substantiallysimultaneously with the projection of ions of opposite sign of thecharging ion source therethrough where discharge and partiallydischarged.

Otherwise, and even more preferable, as for suitable conventionalmaterials, the screen may be charged by an applied field during exposureto the light image and ions of the opposite sign projected therethroughwhere discharged by the light image, as in FIG. 6a, to produce positiveimages.

Effective field blocking of ions requires a combination of high chargelevel and suitable insulator thickness. The range of photosensitivematerials which may be used for the insulator layer can be extended byspecial screen configurations. If the desired insulator material 101(FIG. 40) has low dielectric strength, thus limiting the degree ofcharge separation it can support, a thin undercoating 103 of a highdielectric stength, but not necessarily photoconductive, material can beused to separate the photosensitive layer from high field regions nearthe edge of the holes. The conductor 105 is affixed to the undercoating103.

Similarly, a thin overcoating 107 (FIG. 4b) of high resistivity materialcan be used to provide a charge carrier for photoconductors with poorsurface resistivity.

For photoelectric materials, that cannot be deposited in the layersappropriate to this process, the insulating layer may be formed of anygood insulating material which will accept the sensitive material asthin deposit 109 (FIG. 4c). The entire screen, including portions of theconductive layer, may be coated.

It is computed that the form of the field within the hole is such that,if a hole is only partially charged, i.e., has not developed sufficientcharge to block, the effects of the charge is to limit the aperture ofthe hole. Partially charged holes are created by reduced exposure duringdischarge, as would occur in gray areas of the image. Thus, gray areasreproduce with reduced apparent aperture, forming a half-tonereproduction of a continuous-tone source.

In FIG. 5 a light-type box is generally shown in dotted outline at 49,housing the equipment for reproduction using the ion principle hereindisclosed. The main conveyor comprises screen 51 made in accordance withone or more of the embodiments described herein. A double layer chargemay be produced by Corotron or corona source 55 which is at anadjustable potential E to spray the photoconductor side 50 of screen 51with negative ions, thereby effecting a uniform charge on photoconductor50 relative to conductor layer 52. The screen 51 is conveyed in endlessfashion through conventional components, such as the insulated drivedrums or sprockets 55, and the motion may be intermittent orsynchronized with the paper conveyor 57.

The uniformly double charged screen 51 moves to the image station 59where a conventional scanner 61 or image projecting arrangement shineslight in accordance with the image to discharge screen 51 in such a wayas to modulate the apertures thereof, some apertures being dischargedcompletely, some partially, and some not at all, in accordance with theimage to be reproduced.

The electrostatic latent image in the form of modulated screen aperturesis then conveyed to the ion projections source 63, which may comprise abank of Corotrons 65 disposed quite closely to the interior, i.e.,conductive layer of screen 51. The bank of Corotrons 65 projectsnegative ions toward the grounded, as by roller 66, or positivelycharged side (the conductive layer) to permit the ions to pass throughscreen 51 in accordance with the potential distribution across themodulated apertures. Here again conventionally available controls may beemployed for intermittently or continuously discharging the bank ofCorotrons 65.

It will be seen that the voltage E is of such polarity as to causenegative ions to be propelled toward and through the screen, thusproviding an opposite charge relationship, i.e., positive charges on theside of screen toward which the negative ions are projected. The voltagesource E, may be of the order of 8,000 volts d.c.

The schematic arrangement of FIGS. 5, 6, and 7 may be built generallyusing components selected from the apparatus and control circuity of US.Ser. No. 565,284 in the name of Samuel B. McFarlane, Jr. filed July 14,1966, and entitled Method and Apparatus of Electrostatic ColorReproduction or the US. application Ser. No. 673,499 entitled ModulatedAperture Control for Electrostatic Printing to Gerald Pressman, andassigned to the same assignee as the subject invention. Exposure andprinting are preferably carried out with the conveyor intermittentlystopped, although exposure may be accomplished in line-by-line fashionon a continuous basis and printing done as above described. Similarly,sequential color reproduction may be achieved with the present inventionin accordance with the apparatus disclosed herein identified and as inthe McFarlane application.

In the apparatus of FIGS. 5, 6 and 7, all fields depicted are preferablydirect (d.c.) potential fields, and all may be derived from a commonpower supply.

The arrangement of FIG. 5 is preferably as shown, with the isolatedpaper or print receiving material carriers, shown as conveyors 57 and67, electrically isolated as by a polystyrene or plastic spacer 69,which enables the conveyors 57 and 67 to be at different potentials. Inconveyor 57 the drums 71 and 73 are preferably insulated, and a positivepotential from source E e.g., adjustable from 0 to 3,000 volts, is meshor other conductive material. The paper or other print receivingmaterial 77 may be caused to adhere to the conveyor 57 by vacuum or edgeholding means, not shown, or it may be disposed beneath conveyor 51, asshown in FIGS. 6 and 7.

With the source E applying a positive potential to the carrier orconveyor 57, it will be seen that the field projecting the negative ionsin accordance with voltage source E is enhanced and the source ispreferably adjustable for setting in accordance with the environment,i.e., humidity and other factors which affect potential distributionacross the gap 81.

Once the charge pattern is produced on the print receiving material 77with its non-conductive coating, the material or paper 77 is transferredto conveyor 67 where charged marking powder is caused to be attractedthereto due to the revolving brush 83 and conventional powder chargingsource E Since the pattern is comprised of negative charges, the powderis positively charged by source E, for better adherence. Also, the powerholder compartment 85 may include a screen or mesh 87 across its topthrough which the powder particles are projected. This screen may bemaintained at a positive potential by a source E to further enchance theprojection or powdering field. Also, source E connected to roller 89 mayestablish a negative potential level for conveyor 67, supported byinsulated sprockets or drums 91 and 93 in order further to improve thepowdering action. The potential of these latter sources is not criticaland may be adjustable in the range of 500 to l,000 volts.

In this embodiment fixing is carried out by conventional means withheater source E supplying heating coils or resistors 95 to fix the resinbinder of whatever printing material is employed. A wedge 97 serves todrop the printed sheets 77 onto a further conveyor 98 for exit from thelight tight housing 49. Fixing may be accomplished also by means of aliquid toner, a bead cascade, an aerosol cloud, a magnetic brush, or thelike.

In FIG. 6, there is disclosed an embodiment wherein exposure and ionprojection may take place simultaneously or substantially at the sametime or flash exposure may occur some timer prior to ion projection ifmaterial such as cadmium sulfide and zinc sulfide are used as thephotoconductor layer. In this embodiment, the photoconductor layer 50'comprises the inner side of conveyor 51 with the metal or conductivescreen layer 52' being exposed exteriorly of conveyor 51'. A coronasource 53' of negative ions from voltage source E, is supplied to thephotoconductor 50 of conveyor 51', also mounted on insulated drums orwheels 55'. The negatively charged carrier 51' is exposed by scanner 6]at image station 59' by way of the Nesa glass compartment 64, or othertransparent conductive material, which has its inner surface grounded at66. The bank of Corotrons 63 are in this embodiment connected to thepositive side of source E Positive ions are projected toward thenegatively charged conductor layer 50' and pass where permitted to thepaper or print receiving material 77' carried on conveyor 57 which is ata negative potential due to source E connected via roller 75'. Hereagain the operation may be intermittent or continuous, depending uponthe type scanner 61' employed at image station 59' and thesynchronization of conveyors 51' and 57'.

Powdering is effected by rotating brush 83' now connected to negativesource E, with screen 87 also being connected to negative source EFixing is also effected in conventional manner by resistors 95' andsource E The second conveyor 67' is maintained at a positive potentiallevel by E and a vacuum cleaning system comprising brush 100 and exhaustconduit 103 is provided to clean the powder marking material fromconveyor 67'. The wedge 97' is provided to stack the print receivingmaterial in the stack 105. The usual lighttight housing is provided (notshown) as is shown in FIG. 5.

In operation, positive printing is achieved by flashing the image fromscanner 61', either image by image or line by line, only long enough todischarge the negative charges in the areas to be printed, thuseliminating or diminishing the fringe fields in the image light regionsrelative to the dark regions.

A timer, shown as synchronizer timer 101, which may comprise acommerically available counter, such as manufactured by Eagle SignalDivision of E. W. Bliss Co., is provided to flash the scanner 61 and toapply the voltage source such as E, and E, in proper sequence. Since thephotocopy unit is in a light-tight housing, it is not necessary that thescanner 6] flash the image through the location of the Nesa glass 64',but only that if flash the image onto the negatively charged conveyor51' subsequent to energization of Corotron 53' The light discharges theregions where it impinges, thereby eliminating the fringing fields atthe apertures illuminated. Then the bank of Corotrons 63' projectspositive ions toward the screen and they pass in the illuminated areasto the paper 77 to cause the charge pattern buildup in accordance withthe original scan. This may be achieved in line by line fashion, but isillustrated in image by image reproduction; for this arrangement anintermittent drive (not shown) is provided, also under the control ofsynchronizer timer 101.

The other positive printing method and apparatus is shown in FIG. 6a,which would also incorporate the conveyors 57' and 67' and associatedpowdering and cleaning equipment, as shown in FIG. 6. In FIG. 6a thephotoconductive screen is also on the inside, as shown at 50" of thescreen 5] with the metal screen 52" facing outwardly. The scanner 61focuses the image onto the photoconductive side 50"of the screen 51" viatransparent electrode 106 which is at the potential E in this case shownas a positive potential, and conductor layer 52" is grounded at 107. Themagnitude of source E may be selected from a wide range because the onlypurpose of this source is to establish an applied or internal fieldwhich can be ruptured by image light spots impinging on thephotoconductor to permit a double layer charge buildup. This chargebuildup is trapped by removal of source E because of the dark housinginterior, and accordingly the charge pattern is then conveyed to thebank of Corotrons 63" conveniently disposed in Nesa glass housing 64".

Depending upon the materials employed and particularly the type junctionbetween layers 50" and 52", a positive or negative charge can be builtup on photoconductor 50" where exposed to the light. If zinc oxide isemployed as the photoconductor material, the charge will be negative andwith, for example, selenium as a photoconductor material, it will bepositive.

In either event, the principle obtains that the charge pattern producedby the flash of light removes fringing fields only where the lightimpinged, because the photoconductor becomes conductive to leak off thecharges.

For example, assume that the materials of the screen are such that thepositive transparent electrode 106 induces a charge separation with thenegative charge disposed on the inside of screen 51, Le, on thephotoconductor, and the positive charges are on the opposite side. Thenthe fringing fields will be oriented in the direction from outside theapertures to the inside, or outside the conveyor 51 to the interiorthereof. The light spots remove these charges such that positive ionsfrom source E supplied by Corotron bank 63" will pass through the areasthat were unblocked by the light, but will be diverted by the fringingfields in the dark re gions. Thus, a charge pattern will be laid down onthe paper, such as shown in FIG. 6 at 77', which is subsequentlydeveloped and fixed.

The synchronizer timer may operate the neutralizer 110 continuously orintermittently as desired. The purpose of neutralizer 110 is simply toremove charges from conveyor 51" and it may merely consist of a knownradiation source. Next, synchronizer timer 101 turns on the field fromsource E and then flashes scanner 6] just long enough to remove thecharges where the light impinges. Thereafter, synchronizer timer 101"energizes the Corotrons 63" from source E to project the ions.

In FIG 7, there is disclosed a further embodiment of the inventionwherein the scanner 61a is disposed within the confines of the conveyor51a which has a configuration to locate the photoconductor 50ainteriorly and the conductor layer 52a exteriorly, the latter beinggrounded at 110. The belt or screen 51a is charged negatively by coronawire or Corotron 53a from source E and the photoconductive layer 500 isexposed by scanner 61a being conveyed by insulated sprockets or dums55a. The Corotron bank 630 is maintained at a positive potential E withthe shields grounded at 115. The paper conveyor 57a is maintained at anegative potential by roller contactor a from source E whereby the paper770 is caused to develop a positive charge pattern in accordance withthe image. Isolated conveyor 67a, separated by insulator 69a, ismaintained at a positive potential by E and the powder brush 83a is at anegative potential due to source E with screen 87a also being at anegative potential via source E Otherwise the system is the same as thatdescribed in FIG. 6.

In the various embodiments, the ions employed and the polarities of thevoltage sources are determined by the materials used, but one principleobtains: namely, that the fringing fields block when they are orientedin the direction to oppose the ions. Thus, if positive ions areprojected toward the screen from the inside, a fringing field wouldoppose such ions if its direction were from outside the screen towardthe inside thereof. With this principle in mind, the photoconductor caneither be inside or outside the screen and the source of ionsaccordingly disposed.

Since further modifications of the invention within the principlesherein taught may readily occur to those skilled in the art, it isintended that the invention be limited only by the appended claimswherein:

What is claimed is:

1. Apparatus for electrostatic reproducing comprising in combinationcomposite screen means supported for movement along a predeterminedpath, said screen means comprising an insulative screen layer and aconductive screen layer; means for electrically charging the insulativelayer with substantially equal and opposite charges; means for modifyingthe charges in accordance with an image to be reproduced; means fordirecting ions toward the screen means; means for locating printreceiving material across an air gap in the direction of projection ofthe ions to receive a charge pattern in the form of said image; andmeans for developing the image.

2. The apparatus of claim 1 wherein the means for electrically chargingthe insulative layer comprises a source of ions.

3. The apparatus of claim 1 wherein the means for electrically chargingthe insulative layer comprises an electrode adjacent to the screen meansand a connec tion to the conductive layer whereat a source of electricpower is adapted to be connected between the con nection and theelectrode.

4. The apparatus of claim 3 further comprising synchronizer timer meansfor connecting and disconnecting the electric power source.

5. An aperture-controlled electrostatic printing system comprising incombination a multi-layer screen comprising at least a screen insulatorlayer overlying a screen conductor layer; means for electrostaticallycharging the insulator layer to provide a double charge layer; means formodifying the charges of the insulator layer in accordance with theimage to be reproduced; means for directing ions through the multi-layerscreen in accordnace with the modified charge; means for locating aprint receiving medium oppositely of the insulator layer relative to theaccordance for directing ions, to receive the ions as a charge pattern;and means for developing the charge pattern to produce the image on theprint-receiving substrate.

6. A system for image reproduction through the use of modulatedapertures comprising in combination a multi-layer screen comprising atleast a conductor screen and an insulator screen overlying the conductorscreen and affixed thereto with the apertures coinciding; image means;means for double charging the insulator screen in accordance with theimage means; means for directing ions toward the screen for passagethercthrough in accordance with said charging; and means forintercepting the ions passing through the screen on print-receivingmaterial.

7. Apparatus for electrostatic reproducing comprising in combinationcomposite screen means supported for movement along a predeterminedpath, said screen means comprising an insulative screen layer and aconductive screen layer; a source of ions of a first polarity forcharging the insulative screen layer; an imaging station for shininglight in accordance with an image to be reproduced onto the chargedinsulative screen layer; a source of ions of a polarity opposite saidfirst polarity for projecting ions through the screen means wherepermitted by the diminished or absence of charges thereon; a conveyorfor presenting print receiving material oppositely of saidlast-mentioned source of ions and across an air gap therefrom whereby acharge pattern is laid down on the print receiving material; means fordeveloping the charge pattern; and means for fixing the developer in theform of the pattern.

8. The apparatus of claim 7 further comprising a source of potentialconnected to the print receiving conveyor of polarity opposite to thatof said lastmentioned source of ions to enhance the projection.

9. The apparatus of claim 8 further comprising powder charging meansincluding a source of potential of opposite polarity to the charge laiddown as the charge pattern to enhance developing.

10. Apparatus for electrostatic reproducing comprising in combinationcomposite screen means supported for movement along a predeterminedpath, said screen means comprising an insulative screen layer and aconductive screen layer; a source of ions of a first polarity forcharging the insulative screen layer; an imaging station for flashinglight in accordance with an image to be reproduced onto the chargedinsulative screen layer; a source of ions of a polarity opposite to saidfirst polarity disposed at the imaging station for projecting ionsthrough the screen means where permitted by the diminished or absence ofcharges thereon; said source of ions comprising a voltage source of apolarity to produce ions of opposite charge to said first polarity;synchronizer timer means for controlling the imaging station to flashthe light and for connecting said source of voltage to project theopposite polarity ions; conveyor means for presenting print receivingmaterial oppositely of said last-mentioned source of ions and across anair gap therefrom whereby a charge pattern is laid down on the printreceiving material; means for developing the charge pattern; and meansfor fixing said developer on the print-receiving material.

1 1. Apparatus for electrostatic reproducing comprising in combinationcomposite screen means supported for movement along a predeterminedpath, said screen means comprising an insulative screen layer and aconductive screen layer, an electrode disposed near the screen means,means for applying an electric potential between said electrode and theconductive layer of said screen means for charging the insulative screenlayer; image means for shining light on the insulative layer during theapplication of the charging means to discharge areas where lightimpinges; means for projecting ions toward the screen means to passtherethrough in the regions discharged; means for presenting printreceiving material to the ions passing through the screen means toproduce a charge pattern thereon; means for developing said chargepattern; and synchronizer timer means for controlling the application ofsaid charging electric means and the shining of the image light duringthe application of the charging electric means.

12. The apparatus of claim 11 further comprising neutralizer means forneutralizing any charges on the screen means.

13. The method of electrostatic printing comprising the steps ofapplying an electric field to a combination screen having anelectrically active photosensitive apertured insulative layer and aconductive layer with coinciding apertures and then exposing theinsulative layer to a light image to produce a charge separation acrossthe insulative layer in accordance with the image by maintaining thepotential of the conductive layer fixed; removing the light image andthen the field to trap the charge separation; disposing dielectric printreceiving material adjacent to the screen; directing ions through thescreen in accordance with the image to produce a charge pattern on theprint receiving material; and developing the charge pattern.

14. The method of electrostatic printing comprising the steps ofcharging an insulating screen having an apertured insulative layer and acoinciding apertured conductive layer with a uniform double layer ofcharges comprising charges of opposite polarity on either side of theinsulative layer to establish blocking fields in the apertures of thescreen; modifying the double layer of charges in accordance with animage to be reproduced to electrically unblock apertures thereof inaccordance with the image; directing ions toward the screen to passtherethrough in accordance with the unblocked apertures; interceptingthe ions which pass through the screen on dielectric print receivingmaterial to form a charge pattern; and developing the charge pattern toproduce printing.

15. [n electrostatic printing wherein a multi-layer screen having anelectrically active photosensitive apertured insulative layer and acoinciding apertured conductive layer is used, the steps of electricallycharging the insulative layer while maintaining the conductive layer ata fixed potential to establish fringing fields in the screen apertures;modifying the charge on the insulating layer by shining light inaccordance with an image to be reproduced thereon to at least partlyelimi nate fringing fields where the light impinged; projecting ions atthe modified charged area of the screen to permit ion passage throughthe apertures in the regions where the charge and fringing fields aremodified or eliminated; intercepting the ions which pass through thescreen on dielectric print receiving material to produce a chargepattern; and developing the charge pattern to reproduce the image.

16. The method of claim 15 further comprising the steps of flashing theimage light on the screen, and directing the ions toward the screenafter the flashing.

17. The method of claim 15 wherein the image light is flashed on thescreen simultaneously with the direction of ions toward the screen.

18. The method of electrostatic printing using a screen having at leastan apertured photoconductive layer overlaying a conductive layer havingcoinciding apertures comprising the steps of: establishing a uniformpotential level at the conductive layer; establishing a double layercharge separation across the insulator layer in accordance with an imagecomprising charges of opposite polarity on either side of the insulatorlayer to create fringing fields in the apertures as a result of thedouble layer charge separation; projecting ions through the chargesscreen under control of the fringing fields in accordance with theimage; intercepting the ions which have passed through the screen on adielectric print receiving medium; and developing the intercepted ionsfor printing the image.

19. The method of claim 18 comprising the further step of determiningthe charge of the ions to be employed by selecting it for opposition bythe fringing fields whereby the fringing fields block ion passage inproportion to light intensity establishing them.

1. Apparatus for electrostatic reproducing comprising in combinationcomposite screen means supported for movement along a predeterminedpath, said screen means comprising an insulative screen layer and aconductive screen layer; means for electrically charging the insulativelayer with substantially equal and opposite charges; means for modifyingthe charges in accordance with an image to be reproduced; means fordirecting ions toward the screen means; means for locating printreceiving material across an air gap in the direction of projection ofthe ions to receive a charge pattern in the form of said image; andmeans for developing the image.
 2. The apparatus of claim 1 wherein themeans for electrically charging the insulative layer comprises a sourceof ions.
 3. The apparatus of claim 1 wherein the means for electricallycharging the insulative layer comprises an electrode adjacent to thescreen means and a connection to the conductive layer whereat a sourceof electric power is adapted to be connected between the connection andthe electrode.
 4. The apparatus of claim 3 further comprisingsynchronizer timer means for connecting and disconnecting the electricpower source.
 5. An aperture-controlled electrostatic printing systemcomprising in combination a multi-layer screen comprising at least ascreen insulator layer overlying a screen conductor layer; means forelectrostatically charging the insulator layer to provide a doublecharge layer; means for modifying the charges of the insulator layer inaccordance with the image to be reproduced; means for directing ionsthrough the multi-layer screen in accordnace with the modified charge;means for locating a print receiving medium oppositely of the insulatorlayer relative to the accordance for directing ions, to receive the ionsas a charge pattern; and means for developing the charge pattern toproduce the image on the print-receiving substrate.
 6. A system forimage reproduction through the use of modulated apertures comprising incombination a multi-layer screen comprising at least a conductor screenand an insulator screen overlying the conductor screen and affixedthereto with the apertures coinciding; image means; means for doublecharging the insulator screen in accordance with the image means; meansfor directing ions toward the screen for passage therethrough inaccordance with said charging; and means for intercepting the ionspassing through the screen on print-receiving material.
 7. Apparatus forelectrostatic reproducing comprising in combination composite screenmeans supported for movement along a predetermined path, said scrEenmeans comprising an insulative screen layer and a conductive screenlayer; a source of ions of a first polarity for charging the insulativescreen layer; an imaging station for shining light in accordance with animage to be reproduced onto the charged insulative screen layer; asource of ions of a polarity opposite said first polarity for projectingions through the screen means where permitted by the diminished orabsence of charges thereon; a conveyor for presenting print receivingmaterial oppositely of said last-mentioned source of ions and across anair gap therefrom whereby a charge pattern is laid down on the printreceiving material; means for developing the charge pattern; and meansfor fixing the developer in the form of the pattern.
 8. The apparatus ofclaim 7 further comprising a source of potential connected to the printreceiving conveyor of polarity opposite to that of said last-mentionedsource of ions to enhance the projection.
 9. The apparatus of claim 8further comprising powder charging means including a source of potentialof opposite polarity to the charge laid down as the charge pattern toenhance developing.
 10. Apparatus for electrostatic reproducingcomprising in combination composite screen means supported for movementalong a predetermined path, said screen means comprising an insulativescreen layer and a conductive screen layer; a source of ions of a firstpolarity for charging the insulative screen layer; an imaging stationfor flashing light in accordance with an image to be reproduced onto thecharged insulative screen layer; a source of ions of a polarity oppositeto said first polarity disposed at the imaging station for projectingions through the screen means where permitted by the diminished orabsence of charges thereon; said source of ions comprising a voltagesource of a polarity to produce ions of opposite charge to said firstpolarity; synchronizer timer means for controlling the imaging stationto flash the light and for connecting said source of voltage to projectthe opposite polarity ions; conveyor means for presenting printreceiving material oppositely of said last-mentioned source of ions andacross an air gap therefrom whereby a charge pattern is laid down on theprint receiving material; means for developing the charge pattern; andmeans for fixing said developer on the print-receiving material. 11.Apparatus for electrostatic reproducing comprising in combinationcomposite screen means supported for movement along a predeterminedpath, said screen means comprising an insulative screen layer and aconductive screen layer, an electrode disposed near the screen means,means for applying an electric potential between said electrode and theconductive layer of said screen means for charging the insulative screenlayer; image means for shining light on the insulative layer during theapplication of the charging means to discharge areas where lightimpinges; means for projecting ions toward the screen means to passtherethrough in the regions discharged; means for presenting printreceiving material to the ions passing through the screen means toproduce a charge pattern thereon; means for developing said chargepattern; and synchronizer timer means for controlling the application ofsaid charging electric means and the shining of the image light duringthe application of the charging electric means.
 12. The apparatus ofclaim 11 further comprising neutralizer means for neutralizing anycharges on the screen means.
 13. The method of electrostatic printingcomprising the steps of applying an electric field to a combinationscreen having an electrically active photosensitive apertured insulativelayer and a conductive layer with coinciding apertures and then exposingthe insulative layer to a light image to produce a charge separationacross the insulative layer in accordance with the image by maintainingthe potential of the conductive layer fixed; removing the light imageand then the field to trap the charGe separation; disposing dielectricprint receiving material adjacent to the screen; directing ions throughthe screen in accordance with the image to produce a charge pattern onthe print receiving material; and developing the charge pattern.
 14. Themethod of electrostatic printing comprising the steps of charging aninsulating screen having an apertured insulative layer and a coincidingapertured conductive layer with a uniform double layer of chargescomprising charges of opposite polarity on either side of the insulativelayer to establish blocking fields in the apertures of the screen;modifying the double layer of charges in accordance with an image to bereproduced to electrically unblock apertures thereof in accordance withthe image; directing ions toward the screen to pass therethrough inaccordance with the unblocked apertures; intercepting the ions whichpass through the screen on dielectric print receiving material to form acharge pattern; and developing the charge pattern to produce printing.15. In electrostatic printing wherein a multi-layer screen having anelectrically active photosensitive apertured insulative layer and acoinciding apertured conductive layer is used, the steps of electricallycharging the insulative layer while maintaining the conductive layer ata fixed potential to establish fringing fields in the screen apertures;modifying the charge on the insulating layer by shining light inaccordance with an image to be reproduced thereon to at least partlyeliminate fringing fields where the light impinged; projecting ions atthe modified charged area of the screen to permit ion passage throughthe apertures in the regions where the charge and fringing fields aremodified or eliminated; intercepting the ions which pass through thescreen on dielectric print receiving material to produce a chargepattern; and developing the charge pattern to reproduce the image. 16.The method of claim 15 further comprising the steps of flashing theimage light on the screen, and directing the ions toward the screenafter the flashing.
 17. The method of claim 15 wherein the image lightis flashed on the screen simultaneously with the direction of ionstoward the screen.
 18. The method of electrostatic printing using ascreen having at least an apertured photoconductive layer overlaying aconductive layer having coinciding apertures comprising the steps of:establishing a uniform potential level at the conductive layer;establishing a double layer charge separation across the insulator layerin accordance with an image comprising charges of opposite polarity oneither side of the insulator layer to create fringing fields in theapertures as a result of the double layer charge separation; projectingions through the charges screen under control of the fringing fields inaccordance with the image; intercepting the ions which have passedthrough the screen on a dielectric print receiving medium; anddeveloping the intercepted ions for printing the image.
 19. The methodof claim 18 comprising the further step of determining the charge of theions to be employed by selecting it for opposition by the fringingfields whereby the fringing fields block ion passage in proportion tolight intensity establishing them.